When power switching modules, e.g., IGBT modules/devices, are connected in series and switched, the voltage measured from collector to emitter of each device should not exceed the maximum rating at any time. Considering a static off case however, due to parameter imbalance between modules, the voltage across a device may not remain constant when the series devices are off since the output capacitances of the modules may charge and discharge. It is therefore desirable to take measures to ensure the voltage across each device stays within the module rating when the device is off.
So-called “static voltage sharing” generally maintains a balance of voltages across each individual IGBT module that is connected in series such that no one device exceeds its voltage rating when the devices are off.
Similarly, it is desirable to provide “dynamic voltage sharing” during a switching edge. If a chain of devices doesn't switch synchronously for example because one device switches early or late then a device may be destroyed. For example, as devices turn on it is possible that one device is left behind and it will very quickly be subjected to the full blocking voltage. Dynamic voltage sharing generally maintains a balance of voltages across each individual IGBT module that is connected in series so that no one device exceeds it voltage rating during switching.
One technique for voltage sharing involves using a series of voltage balancing resistors in parallel with the power switching device. Some capacitance devices may also be used in parallel to dominate IGBT module capacitance. Such additional circuitry however can be large and waste power.
Other techniques are generally feedback control schemes. For example, Active Voltage Control (AVC) wherein each power switching device has its own feedback loop, such loops generally being driven with the same reference signal profile. Such circuits may prevent series IGBTs operating beyond their safe operating limits by limiting IGBT collector voltage. FIGS. 14a and 14b respectively represent example an Active Voltage Control (AVC) circuit and a Cascade Active Voltage Control (CAVC) circuit, each showing a reference signal being input to a difference amplifier Vref terminal. Such AVC or CAVC circuitry may be applied to each power switching device in a series connection of such devices. Examples of AVC are found in WO 97/43832 and WO 2008/032113 (both Patrick Palmer et al). (C)AVC can further be used for voltage clamping of a power switching device.
Further regarding AVC, and having considered background art such as discussed above, the present inventors now consider that practical realization of feedback control schemes is difficult due to the parasitic (stray) inductance and resistance in large power switch modules. The inventors have further considered in this regard that to synchronize multiple IGBTs with AVC, a voltage plateau at turn-off may be introduced. This may hold the multiple IGBTs in the linear region for a short time in order to synchronize the switching edge. Further still, the inventors now disclose that at turn-on a current measurement and feedback scheme would be preferable as the current builds up before the voltage changes, so to synchronize devices it may be preferable to hold multiple IGBTs in a low current state. However, current based control may be expensive, inaccurate, lossy and/or the current may be slow to measure. In particular, measuring very high currents (e.g., 1000 s of Amps) without power loss and/or with wide dynamic range is difficult. Furthermore feedback loop delay, which reduces the loop bandwidth, means it may only be possible to control slowly switching devices. Slower switching generally allows more control but also results in higher power consumption, even when the circuit is designed to switch at zero volts.
For certain applications (e.g., High Voltage Direct Current (HVDC) converters and medium voltage motor drives) a solution for series connected IGBT modules is particularly desirable. For example, in a HVDC voltage sourced converter (VSC), series connected IGBT modules can be used as AC switches where the switching frequency is low (100 to 120 times a second). Losses in the system are predominantly conduction losses of the IGBTs, so switching slowly may not be a problem Under certain fault conditions however, the IGBTs are required to switch at a higher rate (say 2 kHz) for a short time, so a system design needs to take account of the higher switching loss under a fault condition The higher frequency switching requirement generally means that other power semiconductors such as thyristors or GTOs cannot be used. Therefore, improved control of power switching devices such as IGBTs is desired.
In view of the above, there remains a need to provide voltage balancing and/or clamping of series connected power switching devices such as IGBTs, with e.g. low power dissipation, accuracy of voltage sharing, and/or fast balancing etc. Additionally or alternatively, improved voltage balancing in static off states and/or dynamic on/off switching transitions of the series connected devices is desirable, preferably with low component count and/or low cost etc, for such voltage balancing and/or voltage clamping.
For use in understanding the present invention, the following disclosures are referred to:                EP 0 898 811 B1 (Cambridge University Technical Services Ltd., Palmer et al), from EP 97 921 962.3 published Mar. 3, 1999 of same family as WO 97/43832, published Nov. 20, 1997, applicants and inventors Patrick Palmer et al.;        US 2010/0060326 A1 (Cambridge Enterprise Ltd., Palmer et al), published Nov. 3, 2010 of same family as WO 2008/032113, published Mar. 20, 2008, applicants and inventors Patrick Palmer et al.;        GB 2 488 778 B (Amantys Ltd., Palmer et al), corresponding to GB 1 103 806.4 published Dec. 9, 2012;        US 2005/0253165 A1 (Pace and Robbins), published Nov. 17, 2005.        